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Three pipes P, Q, R can fill a tank in 4, 8 and 12 hours respectively, while another pipe S can drain it in 10 hours. Which arrangement will fill the tank in the least possible time?
  • a)
    Q alone is opened
  • b)
    P and S are opened together
  • c)
    P, R and S are opened together
  • d)
    Q, R and S are opened together
Correct answer is option 'C'. Can you explain this answer?
Verified Answer
Three pipes P, Q, R can fill a tank in 4, 8 and 12 hours respectively,...
Suppose the volume of the tank is 120 litres. In 1 hour, P, Q and R can fill 30, 15 and 10 litres respectively while S can empty 12 litres. From this, the effective per hour rates(putting the values in option),
for eg in option pipe
Q = 15,
P and S = 30 - 12 = 18.
P, R, S = 30 + 10 - 12 = 28.
Q, R, S = 15 + 10 - 12 = 13.
Thus for answer choices a to d, the rates are are 15, 18, 28 and 13 litres per hour respectively.Thus if the combination from the option are used to find the time i.e 120/rate, the times taken will be 8, 6.66, 4.28 and 9.23 hours respectively. Hence option C.
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Most Upvoted Answer
Three pipes P, Q, R can fill a tank in 4, 8 and 12 hours respectively,...
To find the arrangement that will fill the tank in the least possible time, we need to compare the rates at which each pipe fills or drains the tank. The rate at which a pipe fills or drains the tank can be calculated by taking the reciprocal of the time taken by the pipe to fill or drain the tank.

Let's calculate the rates for each pipe:

- Pipe P can fill the tank in 4 hours, so its filling rate is 1/4 tanks per hour.
- Pipe Q can fill the tank in 8 hours, so its filling rate is 1/8 tanks per hour.
- Pipe R can fill the tank in 12 hours, so its filling rate is 1/12 tanks per hour.
- Pipe S can drain the tank in 10 hours, so its draining rate is -1/10 tanks per hour (negative sign indicates draining).

Now, let's consider each arrangement and calculate the overall rate for each:

a) Q alone is opened:
The overall rate for this arrangement is 1/8 tanks per hour, as Q is the only pipe opened.

b) P and S are opened together:
The overall rate for this arrangement is (1/4 - 1/10) tanks per hour, as P fills the tank and S drains the tank. Simplifying this, we get 3/20 tanks per hour.

c) P, R, and S are opened together:
The overall rate for this arrangement is (1/4 + 1/12 - 1/10) tanks per hour, as P and R fill the tank and S drains the tank. Simplifying this, we get 1/6 tanks per hour.

d) Q, R, and S are opened together:
The overall rate for this arrangement is (1/8 + 1/12 - 1/10) tanks per hour, as Q and R fill the tank and S drains the tank. Simplifying this, we get 7/120 tanks per hour.

From the calculations above, we can see that option c) P, R, and S opened together has the highest overall rate of 1/6 tanks per hour. Therefore, this arrangement will fill the tank in the least possible time.
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Three pipes P, Q, R can fill a tank in 4, 8 and 12 hours respectively, while another pipe S can drain it in 10 hours. Which arrangement will fill the tank in the least possible time?a)Q alone is openedb)P and S are opened togetherc)P, R and S are opened togetherd)Q, R and S are opened togetherCorrect answer is option 'C'. Can you explain this answer?
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